Fossil Fissile Fuels

Nils Johan Engelsen
March 17, 2011

Fig. 1: Plot of binding energy per nucleon vs.
nucleon number for all known isotopes. Data from [4].

Introduction

Recent focus on carbon dioxide emissions has led to
nuclear energy once again being considered for electricity generation.
[1-3] The previous science and innovation minister of Britain even moved
to reclassify nuclear power as renewable, but is it really so outside of
parliament? [4] Current commercial reactors use Uranium as fuel, and
while Uranium has a fairly high average abundance of 2.7 parts per million in the
Earth's crust, there are few economically viable sources. [5,6] Uranium
is a fossil fuel without the carbon dioxide emissions, but the important
question is how long it will last. As with oil, the lifetime of the
Earth's Uranium supplies is controversial. [5,7] About 16% of the
World's electricity is generated by nuclear reactors and if this
percentage is to increase, the lifetime of Uranium supplies may be
shorter than we think. [8]

Origin of Fissile Fuels

The energy produced in conventional nuclear reactors
comes from fission, where a heavy nuclei is split into two lighter
nuclei. Nuclear fission releases energy because the smaller nuclei have
a higher binding energy per nucleon than heavier nuclei, as can be seen
on Figure 1. The peak in binding energy occurs at iron, which has 56
nucleons. Fission of one U-235 atom releases about 193.9 MeV
of energy, which means that a coffee cup of Uranium (1 kg) can generate
enough energy to power 2000 American homes for a year, assuming perfect
conversion. [11, 12] Similarly, light nuclei like Hydrogen or Lithium
can undergo fusion to release energy, but controlled fusion has not been
realized yet. One might wonder where the enormous amount of energy
contained in these heavy nuclei came from, and it is believed that they
were generated in supernovae explosions. These extremely energetic star
explosions give rise to the conditions needed to make heavy nuclei like
Uranium and Thorium through exotic nuclear reactions. [11] The amount of
Uranium and Thorium on Earth was fixed when the Earth was formed, and
one might consider nuclear fuels less renewable than other fossil fuels,
as oil and coal is generated from organic material continuously.

Mineable Uranium Resources

The most important Uranium reserves are found in
Kazakhstan, Canada and Australia where Uranium is found in high enough
concentrations to be commercially viable at current Uranium prices. Over
half the World's yearly Uranium production originates in these three
countries. [5] Uranium is found in the form of Uranium oxides and is
usually extracted by open-pit mining. The Uranium ore is then purified,
and in most cases enriched to obtain a higher percentage of
U-235. The Uranium can then be used to manufacture fuel rods,
which can finally generate electricity in nuclear reactors. The OECD
estimates that there are 6.3 million tons of identified Uranium supplies
recoverable at a rate less than $260/kg. At 2008 rates of Uranium
consumption, these supplies would last about 100 years. If the
projected discoveries are included as well, world supplies will last 220
years. [5] If Uranium consumption rates increase, as they are projected
to, the supplies will last even shorter. A long-term energy solution can
therefore not be based on minable Uranium in current reactor technology.

Technological Advances

The estimates given above only include mineable
Uranium, and they also assume that there are no improvements in reactor
technology. The most direct way of increasing nuclear fuel supplies
would be to extract Uranium from sea water, where it is found in small
concentrations. Pilot projects in Japan have estimated the price at
200-300$/kg. [12] The total amount of Uranium in the ocean is about 4.5
billion tons, but it remains to be seen whether it can be extracted on a
large scale. Development of breeder reactors where fertile U-238 is
transmuted to fissile Pu-239 in-situ such that more fissile material is
produced than is consumed, would significantly prolong the lifetime of
fissile fuel supplies. The use of the fertile material Thorium in a
Thorium fuel cycle could allow us to use the Earth's Thorium supplies.
Thorium is three to four times more abundant than Uranium and a Thorium
fuel cycle may have significant advantages over the Uranium fuel cycle
currently employed. However, significant technological and economic
challenges remain before Thorium nuclear reactors are commercially
viable and only India has a Thorium nuclear power program. [8]
Reprocessing of spent fuel is a currently available technology
increasing the efficiency of nuclear fuel by extracting the remaining
fissile material in a spent fuel rod by the PUREX method. All major
nuclear powers except the United States have a currently operating
reprocessing program for spent fuel rods. [13]

Conclusions

The fossil nature of nuclear fuel should not be
overlooked when planning for future energy production, but there are
many potential solutions to the fuel problem.